Spinning low fluorosurfactant fluoropolymer dispersions

ABSTRACT

A process for dispersion spinning non-melt-processible fluoropolymer fiber in which a mixture of an aqueous dispersion of non-melt-processible polytetrafluoroethylene or modified polytetrafluoroethylene fluoropolymer particles and an aqueous solution of matrix polymer is formed. The non melt-processible particles have an SSG of less than about 2.40. The aqueous dispersion contains an aliphatic alcohol ethoxylate nonionic surfactant having a 20% residuals temperature determined by thermogravimetric analysis (TGA) of less than about 290° C. and is essentially free of surfactants containing aromatic groups. The dispersion has a fluorinated surfactant content of less than about 300 ppm. The mixture is extruded into a coagulation bath containing a concentration of ions which coagulate the matrix polymer to form an intermediate fiber structure. The intermediate fiber structure is sintered to decompose the matrix polymer and coalesce. The present invention also provides a spinning composition useful for the dispersion spinning of non-melt-processible fluoropolymer fiber.

FIELD OF THE INVENTION

This invention relates to a process for dispersion spinningnon-melt-processible fluoropolymers to form fibers.

BACKGROUND OF THE INVENTION

Polytetrafluoroethylene (PTFE) homopolymers and related modified PTFEpolymers have exceptional stability to light, heat, solvents, chemicalattack and electrical stresses, conferring desirable properties toarticles made from these polymers. But because of the inability to meltprocess these polymers and the difficulties associated with solutionprocessing, it is very difficult to spin or shape them by conventionalmethods. Therefore other processes have been developed for preparingfibers of PTFE homopolymers and modified PTFE.

Dispersion spinning is one method developed for producing shapedarticles such as fibers from fluorinated polymers. Non-melt processiblefluoropolymers may be successfully spun from a mixture of an aqueousdispersion of fluorinated polymer particles mixed with a solution of asuitable matrix polymer. An intermediate structure is formed when thismixture is contacted with a suitable coagulation bath. Although theintermediate structure is mechanically sound, a final, sinteredstructure is generally formed by heating the intermediate structure to atemperature sufficient to coalesce the fluorinated polymer particles. Onsintering the matrix polymer decomposes to form volatile gases and acarbonaceous residue.

Fluorosurfactants are typically used in the dispersion polymerization offluoropolymers, the fluorosurfactants functioning as a non-telogenicdispersing agent, as described in U.S. Pat. No. 2,559,752 to Berry.Unless removed, fluorosurfactant is present in fluoropolymer dispersionsand is present in the fiber spinning compositions made from suchdispersions. However because of environmental concerns, processes havebeen developed to reduce the fluorosurfactant content in aqueousfluoropolymer dispersions to decrease emissions of fluorosurfactantsand/or decrease or eliminate the need to capture fluorosurfactantsduring end use processing of fluoropolymer dispersions.

Several techniques are known for the reduction of fluorosurfactantcontent from fluoropolymer dispersions such as concentration byultrafiltration as described in U.S. Pat. No. 4,369,266 (Kuhls et al.)and contact with ion exchange resin as described in U.S. Pat. No.3,882,153 (Seki et al), U.S. Pat. No. 4,282,162 (Kuhls) and US2003/0125421 A1 (Bladel et al.).

Fluoropolymer dispersions are typically subjected to a concentrationstep after manufacture to increase their concentration. Nonionicsurfactants are usually added prior to concentration to increase thestability of the dispersion. For use in known fiber spinning processes,fluoropolymer dispersions containing alkyl phenol ethoxylates have beenused for stabilizing and concentrating fluoropolymer dispersions.However, alkyl phenol ethoxylates are prone to causing smoking duringsintering of the fiber, causing foaming in wash water, and formingdeposits on rolls which contact the fiber. In addition, because ofenvironmental concerns about compounds containing aromatic groups, it isdesirable to avoid alkyl phenol ethoxylates which contain aromaticgroups.

An improved fluoropolymer spinning process and spinning composition aredesired that employ fluoropolymer dispersions with reducedfluorosurfactant content and which overcome problems associated withalkyl phenol ethoxylate nonionic surfactants.

SUMMARY OF THE INVENTION

The invention is based on the discovery that a fluoropolymer fiberspinning composition of aqueous fluoropolymer dispersion having a lowsurfactant content, containing an aliphatic alcohol ethoxylate nonionicsurfactant having a 20% residuals temperature determined bythermogravimetric analysis (TGA) of less than about 290° C., and beingessentially free of surfactants containing aromatic groups, can providesignificant advantages in producing fluoropolymer fibers.

The present invention provides a process for dispersion spinningnon-melt-processible fluoropolymer fiber in which a mixture of anaqueous dispersion of non-melt-processible polytetrafluoroethylene ormodified polytetrafluoroethylene fluoropolymer particles and an aqueoussolution of matrix polymer is formed. The non melt-processible particleshave an SSG of less than about 2.40. The aqueous dispersion contains analiphatic alcohol ethoxylate nonionic surfactant having a 20% residualstemperature determined by thermogravimetric analysis (TGA) of less thanabout 290° C. and is essentially free of surfactants containing aromaticgroups. The dispersion has a fluorinated surfactant content of less thanabout 300 ppm. The mixture is extruded into a coagulation bathcontaining a concentration of ions which coagulate the matrix polymer toform an intermediate fiber structure. The intermediate fiber structureis sintered to decompose the matrix polymer and coalesce thenon-melt-processible fluoropolymer particles to form the fiber.

The present invention also provides a spinning composition useful forthe dispersion spinning of non-melt-processible fluoropolymer fiber. Thecomposition comprises a mixture of an aqueous solution of a matrixpolymer and an aqueous dispersion of non-melt-processiblepolytetrafluoroethylene or modified polytetrafluoroethylene particleshaving an SSG of less than about 2.40. The aqueous dispersion containsan aliphatic alcohol ethoxylate nonionic surfactant having a 20%residuals temperature determined by thermogravimetric analysis (TGA) ofless than about 290° C. and is essentially free of surfactantscontaining aromatic groups. The dispersion has a fluorinated surfactantcontent of less than about 300 ppm.

In a preferred embodiment of the invention, the 20% residualstemperature determined by TGA of said aliphatic alcohol ethoxylateemployed in the non-melt-processible polytetrafluoroethylene or modifiedpolytetrafluoroethylene dispersion is less than about 285° C., mostpreferably less than about 280° C.

In another preferred embodiment of the invention, the five minute foamheight determined by ASTM D 1173-53 of the aliphatic alcohol ethoxylatesurfactant employed in the non-melt-processible polytetrafluoroethyleneor modified polytetrafluoroethylene dispersion is less than about 100mm, more preferably less than about 50 mm, and most preferably less thanabout 20 mm.

Another embodiment of the present invention provides a process fordispersion spinning non-melt-processible fluoropolymer fiber in which amixture of an aqueous dispersion of non-melt-processiblepolytetrafluoroethylene or modified polytetrafluoroethylenefluoropolymer particles and an aqueous solution of matrix polymer isformed. The non melt-processible particles have an SSG of less thanabout 2.40. The aqueous dispersion contains an aliphatic alcoholethoxylate nonionic surfactant having a thermal decompositiontemperature determined by thermogravimetric analysis (TGA) of less thanabout 250° C. and is essentially free of surfactants containing aromaticgroups. The dispersion has a fluorinated surfactant content of less thanabout 300 ppm. The mixture is extruded into a coagulation bathcontaining a concentration of ions which coagulate the matrix polymer toform an intermediate fiber structure. The intermediate fiber structureis sintered to decompose the matrix polymer and coalesce thenon-melt-processible fluoropolymer particles to form the fiber.

Another preferred embodiment of the present invention provides aspinning composition useful for the dispersion spinning ofnon-melt-processible fluoropolymer fiber. The composition comprises amixture of an aqueous solution of a matrix polymer and an aqueousdispersion of non-melt-processible polytetrafluoroethylene or modifiedpolytetrafluoroethylene particles having an SSG of less than about 2.40.The aqueous dispersion contains an aliphatic alcohol ethoxylate nonionicsurfactant having a thermal decomposition temperature determined bythermogravimetric analysis (TGA) of less than about 250° C. and isessentially free of surfactants containing aromatic groups. Thedispersion has a fluorinated surfactant content of less than about 300ppm.

In a more preferred embodiments of this form of the invention, thethermal decomposition temperature determined by TGA of said aliphaticalcohol ethoxylate employed in the non-melt-processiblepolytetrafluoroethylene or modified polytetrafluoroethylene dispersionis less than about 240° C., most preferably less than about 230° C.

In accordance with a particularly preferred embodiment of the invention,an aqueous dispersion is provided which comprises non-melt-processiblepolytetrafluoroethylene or modified polytetrafluoroethylene particleshaving an SSG of less than about 2.40. The aqueous dispersion containsan aliphatic alcohol ethoxylate nonionic surfactant, the aliphaticalcohol ethoxylate surfactant being an ethoxylate of2,6,8-trimethyl-4-nananol and having a five minute foam heightdetermined by ASTM D 1173-53 of less than about 20 mm.

DETAILED DESCRIPTION

Fluoropolymers

The fluoropolymer particles used in the dispersion employed in thisinvention are non-melt-processible particles of polytetrafluoroethylene(PTFE) including modified PTFE which is not melt-processible.Polytetrafluoroethylene (PTFE) refers to the polymerizedtetrafluoroethylene by itself without any significant comonomer present.Modified PTFE refers to copolymers of TFE with such small concentrationsof comonomer that the melting point of the resultant polymer is notsubstantially reduced below that of PTFE. The concentration of suchcomonomer is preferably less than 1 wt %, more preferably less than 0.5wt %. The modified PTFE contains a small amount of comonomer modifierwhich improves film forming capability during baking (fusing), such asperfluoroolefin, notably hexafluoropropylene (HFP) or perfluoro(alkylvinyl)ether (PAVE), where the alkyl group contains 1 to 5 carbon atoms,with perfluoro(ethyl vinyl) ether (PEVE) and perfluoro(propylvinyl)ether (PPVE) being preferred. Chlorotrifluoroethylene (CTFE),perfluorobutyl ethylene (PFBE), or other monomer that introduces bulkyside groups into the molecule are also included. The PTFE typically hasa melt creep viscosity of at least 1×10⁹ Pa·s. The resins in thedispersion used in this invention when isolated and dried arenon-melt-processible. Such high melt viscosity indicates that the PTFEdoes not flow in the molten state and therefore is non-melt-processible.

By non-melt-processible, it is meant that no melt flow is detected whentested by the standard melt viscosity determining procedure formelt-processible polymers. This test is according to ASTM D-1238-00modified as follows: The cylinder, orifice and piston tip are made ofcorrosion resistant alloy, Haynes Stellite 19, made by Haynes StelliteCo. The 5.0 g sample is charged to the 9.53 mm (0.375 inch) insidediameter cylinder which is maintained at 372° C. Five minutes after thesample is charged to the cylinder, it is extruded through a 2.10 mm(0.0825 inch diameter), 8.00 mm (0.315 inch) long square-edge orificeunder a load (piston plus weight) of 5000 grams. This corresponds to ashear stress of 44.8 KPa (6.5 pounds per square inch). No melt extrudateis observed.

In one preferred embodiment, the fluoropolymer particles in thedispersion used in this invention comprise a core of high molecularweight polytetrafluoroethylene (PTFE) and a shell of lower molecularweight polytetrafluoroethylene or modified polytetrafluoroethylene.

The fluoropolymer particles have a standard specific gravity (SSG) ofless than 2.40, typically from about 2.14 to about 2.40, preferably lessthan about 2.30, and more preferably less than about 2.25. The SSG isgenerally inversely proportional to the molecular weight of PTFE ormodified PTFE.

The fluoropolymer particles in the dispersion used in this inventionpreferably have a number average particle size of about 100 nm to about400 nm, most preferably, about 120 nm to about 220 nm.

The fluoropolymer dispersion used in this invention is made bydispersion polymerization (also known as emulsion polymerization). Atypical process for the aqueous dispersion polymerization of preferredpolymer PTFE is a process wherein TFE vapor is fed to a heated reactorcontaining fluorosurfactants, paraffin wax and deionized water. A chaintransfer agent may also be added if it is desired to reduce themolecular weight of the PTFE. A free-radical initiator solution is addedand, as the polymerization proceeds, additional TFE is added to maintainthe pressure. The exothermic heat of reaction is removed by circulatingcooling water through the reactor jacket. After several hours, the feedsare stopped, the reactor is vented and purged with nitrogen, and the rawdispersion in the vessel is transferred to a cooling vessel. Paraffinwax is removed and the dispersion is isolated and stabilized withnonionic surfactant.

The dispersing agent used in this process is preferably a fluorinatedsurfactant. The fluorosurfactant in the dispersion is a non-telogenic,anionic dispersing agent, soluble in water and comprising an anionichydrophilic group and a hydrophobic portion. Preferably, the hydrophobicportion is an aliphatic fluoroalkyl group containing at least fourcarbon atoms and bearing fluorine atoms and having no more than twocarbon atoms not bearing fluorine atoms adjacent to the hydrophilicgroup. These fluorosurfactants are used as a polymerization aid fordispersing and, because they do not chain transfer, they do not causeformation of polymer with undesirable short chain length. An extensivelist of suitable fluorosurfactants is disclosed in U.S. Pat. No.2,559,752 to Berry. Preferably, the fluorosurfactant is a perfluorinatedcarboxylic or sulfonic acid having 6-10 carbon atoms and is typicallyused in salt form. Suitable fluorosurfactants are ammoniumperfluorocarboxylate, e.g., ammonium perfluorocaprylate or ammoniumperfluorooctanoate. The fluorosurfactants are usually present in theamount of 0.02 to 1 wt % with respect to the amount of polymer formed.The fluorinated surfactant is used to aid the polymerization process butthe amount remaining in the concentrated dispersion composition used inthe fiber spinning process is significantly reduced as will be explainedbelow.

The initiators preferably used to make dispersion for use in the processof this invention are free radical initiators. They may be those havinga relatively long half-life, preferably persulfates, e.g., ammoniumpersulfate or potassium persulfate. To shorten the half-life ofpersulfate initiators, reducing agents such as ammonium bisulfite orsodium metabisulfite, with or without metal catalysis salts such as Fe(III), can be used. Alternatively, short half-life initiators such aspotassium permanganate/oxalic acid can be used.

In addition to the long half-life persulfate initiators, small amountsof short chain dicarboxylic acids such as succinic acid or initiatorsthat produce succinic acid such as disuccinic acid peroxide (DSP) may bealso be added in order to reduce coagulum

To produce dispersion with low fluorosurfactant content as describedbelow, sufficient nonionic surfactant is added to prevent coagulation ofthe dispersion when the fluorosurfactant content is reduced. Typically,nonionic surfactant is added for stabilization prior to fluorosurfactantreduction and then as desired, concentration of the dispersion isconducted. For concentrating, the polymer is held at a temperature abovethe cloud point of the nonionic surfactant. Once concentrated to about30 to about 70 weight % fluoropolymer, and preferably about 45 to about65 weight % fluoropolymer, the upper clear supernate is removed. Furtheradjustment of the final solids concentration and surfactant are made asneeded. One patent illustrative of concentrating is U.S. Pat. No.3,037,953 to Marks and Whipple.

Nonionic Surfactants

Nonionic surfactants used in dispersions employed in accordance with theinvention are aliphatic alcohol ethoxylates. They are preferably presentin the dispersion in amounts of about 2 to about 11 weight %, mostpreferably about 3 to about 11 weight %, based on the weight of saidfluoropolymer. Suitable nonionic surfactants include any of a variety ofaliphatic alcohol ethoxylates or mixtures thereof which provide thedesired cloud point during concentration. The aliphatic alcoholethoxylates employed in carrying out the present invention also have a20% residuals temperature determined by TGA of less than about 290° C.,preferably less than 285° C. more preferably less than 280° C. andtypically fall within the preferred range of 250° C. to 290° C. Inaddition or in the alternative, it is preferred that the aliphaticalcohol ethoxylate nonionic surfactant has a thermal decompositiontemperature determined by thermogravimetric analysis (TGA) of less thanabout 250° C., more preferably less than about 240° C., most preferablyless than about 230° C.

Further the dispersions used in this invention are essentially free ofsurfactants containing aromatic groups that can thermally decompose andbe converted to harmful organic aromatic compounds that may adverselyaffect air and water quality during dispersion spinning processes. Inaddition, these materials are prone to producing tar-like buildup onsintering rolls, producing smoke and causing foaming in wash water.Essentially free of essentially free of surfactants containing aromaticgroups preferably means that the dispersions employed contain less thanabout 0.5 weight % of such surfactants. The surfactants used in thisinvention burn off cleanly without thermally decomposing on a substrateleaving lower residuals than alkyl phenol ethoxylates. The preferredalcohol ethoxylate surfactants used in this invention burn off at alower temperature (about 50° C. lower) than the conventional alkylphenol ethoxylates. As will be further discussed below, this lowerburnoff temperature results in a much improved fiber spinning process.Further, a cleaner burnoff that leaves low residuals and avoidscatalyzing polymer decomposition at high application temperatures thusleading to fluoropolymer fiber with higher thermal stability.

In addition the surfactants used in this invention preferably generateless foam during fiber processing. Reduced foam generation can bequantified by the Ross-Miles Foam test measuring five minute foam height(ASTM D 1173-53, reapproved 2001) as detailed below in the Methods forProperty Determination. The five minute foam height for the aliphaticalcohol ethoxylates used in the present invention is preferably lessthan about 100 mm and more preferably less than about 50 mm, and mostpreferably less than about 20 mm. Foam height as shown in the test databelow indicates that fiber spinning compositions containing alkyl phenolethoxylates will retain foam over longer periods of times than aliphaticalcohol ethoxylate nonionic surfactant used in this invention having alower 20% residuals temperature and being essentially free of aromaticgroups. Foam retention is detrimental to fiber processing leading tospinning breaks and causing the fiber manufacturer to take have to taketime-consuming steps to control and reduce foam generation, therebyaffecting productivity.

Especially preferred aliphatic alcohol ethoxylates are a compound ormixture of compounds of the formula:R(OCH₂CH₂)_(n)OHwherein R is a branched alkyl, branched alkenyl, cycloalkyl, orcycloalkenyl hydrocarbon group having 8-18 carbon atoms and n is anaverage value of 5 to 18. For example, a preferred ethoxylate used inthis invention can be considered to be prepared from (1) a primaryalcohol that is comprised of a hydrocarbon group selected from branchedalkyl, branched alkenyl, cycloalkyl or cycloalkenyl or (2) a secondaryor tertiary alcohol. In any event, the ethoxylate used in accordancewith this invention does not contain an aromatic group. The number ofethylene oxide units in the hydrophilic portion of the molecule maycomprise either a broad or narrow monomodal distribution as typicallysupplied or a broader or bimodal distribution which may be obtained byblending.

The cloud point of a surfactant is a measure of the solubility of thesurfactant in water. The surfactants employed in the aqueous dispersionof this invention preferably have a cloud point of about 30° C. to about90° C., preferably about 35° C. to about 85° C.

Nonionic surfactants of the type generally used to stabilizefluoropolymer dispersions can be either liquids or solids at roomtemperature. If solid, the surfactant tends to be pasty and difficult tohandle. They can be handled but often require heated tanks and transferlines to keep them as a liquid. In addition to the capital cost of theheated equipment, there are operational restrictions placed on thesystem. If the temperature is maintained too low, tanks and transferlines can become plugged with solid material. If the temperature is toohigh, degradation of the surfactant can occur.

Generally low viscosity liquids are preferred from a handling point ofview. High viscosity liquids are more difficult to handle and oftenrequire heated tanks and lines to keep the viscosity low enough for easein handling. Some of the apparent liquid surfactants are physicallymeta-stable in that they may exist as liquids for several days and thenturn into pasty solids. Sometimes water is added to the surfactant tolower its viscosity and making it easier to handle. However, too muchwater detracts from the desire to produce more concentrated dispersions.

The aqueous dispersion of non-melt-processible fluoropolymer particlesand nonionic surfactant used in this invention preferably contains anonionic surfactant containing 0-20 weight % water, preferably 0-15weight % water and is a stable liquid at room temperature. A surfactantis considered to be a stable liquid if it remains liquid for 3 days atroom temperature after being chilled to 5° C. and then warmed to roomtemperature (about 23±3° C.).

Dispersions containing nonionic surfactant made as described herein thusare stabilized fluorosurfactant-containing dispersions suitable for usein the reduction of the fluorosurfactant content as will be describedbelow.

Examples of useful aliphatic alcohol ethoxylates surfactants for thisinvention are listed in Tables 1 and 2 and are contrasted to lessdesirable alkyl phenol ethoxylate surfactants, such as Triton® X-100.

In accordance with a particularly preferred embodiment of the invention,an aqueous dispersion is provided which comprises non-melt-processiblepolytetrafluoroethylene or modified polytetrafluoroethylene particleshaving an SSG of less than about 2.40. The aqueous dispersion containsan aliphatic alcohol ethoxylate nonionic surfactant, the aliphaticalcohol ethoxylate surfactant being an ethoxylate of2,6,8-trimethyl-4-nananol and having a five minute foam heightdetermined by ASTM D 1173-53 of less than about 20 mm. Preferably, theaqueous dispersion has a fluorinated surfactant content of less thanabout 300 ppm. It is also preferred for the aqueous dispersion to beessentially free of surfactants containing aromatic groups.

In a more preferred form of this dispersion, the aliphatic alcoholethoxylate nonionic surfactant comprises ethoxylates of2,6,8-trimethyl-4-nananol having an average of about 4 to about 12ethylene oxide (EO) units, most preferably, ethoxylates of2,6,8-trimethyl-4-nananol having an average about 9 to about 11 ethyleneoxide units. Examples of preferred surfactants of this type are thosesold under the trademark Tergitol® TMN-6 (nominally 6 EO units) andTergitol® TMN-10 (nominally 10 EO units).

Fluorosurfactant Reduction

The fluorosurfactant content of the aqueous dispersion ofnon-melt-processible PTFE or modified PTFE particles used in accordancewith the present invention is reduced to a predetermined level of nogreater than about 300 ppm, preferably to a predetermined level nogreater than about 100 ppm, more preferably to a predetermined level nogreater than about 50 ppm.

While any suitable method can be used to reduce fluorosurfactantcontent, contacting the aqueous dispersion with an anion exchange resinis advantageously used for this purpose. Contacting of the dispersionwith anion exchange resin can occur before or after concentration buttypically the lower solids material before concentration is easier toprocess, especially when a fixed bed is employed for carrying out thecontacting step. If the process is carried out prior to concentration,nonionic surfactants are added prior to contact with the anion exchangeresin as discussed above. Further, it is common to add a nonfluorinatedanionic surfactant such as sodium lauryl sulfate to the dispersion priorto concentration to prevent a viscosity increase which can occur uponconcentration. A nonfluorinated cationic surfactant can also be used asdescribed in U.S. Application No. 60/638,310, filed Dec. 22, 2004.

Any of a variety of techniques which bring the dispersion in contactwith the anion exchange resin can be used to carry out ion exchange ofthe process. For example, the process can be carried out by addition ofion exchange resin bead to the dispersion in a stirred tank, in which aslurry of the dispersion and resin is formed, followed by separation ofdispersion from the anion exchange resin beads by filtration. Anothersuitable method is to pass the dispersion through a fixed bed of anionexchange resin instead of using a stirred tank. Flow can be upward ordownward through the bed and no separate separation step is needed sincethe resin remains in the fixed bed.

The contacting of the dispersion is performed at a temperature which issufficiently high to facilitate the rate of ion exchange and to reducethe viscosity of the dispersion but being below a temperature at whichthe resin degrades at a detrimentally high rate or a viscosity increasein observed. Upper treatment temperature will vary with the type ofpolymer and nonionic surfactant employed. Typically, temperatures willbe between 20° C. and 80° C.

The fluorosurfactant can be recovered from the anion exchange resin ifdesired or the resin with the fluorosurfactant can be disposed of in anenvironmentally acceptable method, e.g., by incineration. If it isdesired to recover the fluorosurfactant, the fluorosurfactant may beremoved from resin by elution. Elution of fluorosurfactant adsorbed onthe anion exchange resin is readily achieved by use of ammonia solutionas demonstrated by Seki in U.S. Pat. No. 3,882,153, by a mixture ofdilute mineral acid with organic solvent (e.g., HCl/ethanol) asdemonstrated by Kuhls in U.S. Pat. No. 4,282,162, or by strong mineralacids such as sulfuric acid and nitric, transferring the adsorbedfluorinated carboxylic acid to the eluent. The fluorosurfactant in theeluent in high concentration can easily be recovered in the form of apure acid or in the form of salts by common methods such asacid-deposition, salting out, and other methods of concentration, etc.

Ion Exchange Resins

The ion exchange resins for use in accordance with reducing thefluorosurfactant content of the aqueous dispersion used in the presentinvention include anionic resins but can also include other resin typessuch as cationic resins, e.g., in a mixed bed. The anionic resinsemployed can be either strongly basic or weakly basic. Suitable weaklybasic anion exchange resins contain primary, secondary amine, ortertiary amine groups. Suitable strongly basic anion exchange resincontain quaternary ammonium groups. Although weakly basic resins areuseful because they can be regenerated more easily, strongly basisresins are preferred when it is desired to reduce fluorosurfactant tovery low levels and for high utilization of the resin. Strongly basicion exchange resins also have the advantage of less sensitivity to thepH of the media. Strong base anion exchange resins have an associatedcounter ion and are typically available in chloride or hydroxide formbut are readily converted to other forms if desired. Anion exchangeresins with hydroxide, chloride, sulfate, and nitrate can be used forthe removal of the fluorosurfactant but anion exchange resins in theform of hydroxide are preferred to prevent the introduction ofadditional anions and to increase pH during anion exchange because ahigh pH, i.e., greater than 9, is desirable in the product prior toshipping to inhibit bacterial growth. Examples of suitablecommercially-available strong base anion exchange resins with quaternaryammonium groups with a trimethylamine moiety include DOWEX® 550A, USFilter A464-OH, SYBRON M-500-OH, SYBRON ASB1-OH, PUROLITE A-500-OH,Itochu TSA 1200, AMBERLITE® IR 402. Examples of suitablecommercially-available strong base anion exchange resins with quaternaryammonium groups with a dimethyl ethanol amine moiety include US FilterA244-OH, AMBERLITE® 410, DOWEX® MARATHON A2, and DOWEX® UPCORE Mono A2.

Ion exchange resin used to reduce fluorosurfactant for use in theprocess of the present invention is preferably monodisperse. Preferably,the ion exchange resin beads have a number average size distribution inwhich 95% of the beads have a diameter within plus or minus 100 μm ofthe number average bead diameter.

The monodisperse ion exchange resin has a particle size which provides asuitable pressure drop through the bed. As discussed previously, verylarge beads are fragile and prone to breakage. Very small ion exchangebeads are susceptible to tight particle packing resulting in tortuouschannels in the bed. This can result in high shear conditions in thebed. Preferred ion exchange resin has a number average bead size about450 to about 800 μm, more preferably, the ion exchange resin beads havea number average bead diameter of about 550 to about 700 μm.

Spinning Composition and Matrix Polymers

The present invention provides a spinning composition useful for thedispersion spinning of non-melt-processible fluoropolymer fibercomprising a mixture of an aqueous solution of a matrix polymer and anaqueous dispersion of non-melt-processible polytetrafluoroethylene ormodified polytetrafluoroethylene particles having an SSG of less thanabout 2.40, typically from about 2.14 to about 2.40. The aqueousdispersion containing an aliphatic alcohol ethoxylate nonionicsurfactant and being essentially free of surfactants containing aromaticgroups and the aliphatic alcohol ethoxylate has a 20% residualstemperature determined by thermogravimetric analysis (TGA) of less thanabout 290° C., preferably less than 285° C., most preferably less than280° C. and especially preferred in the range of 250° C. to 290° C. Inaddition or in the alternative, it is preferred that the aliphaticalcohol ethoxylate nonionic surfactant has a thermal decompositiontemperature determined by thermogravimetric analysis (TGA) of less thanabout 250° C., more preferably less than about 240° C., most preferablyless than about 230° C. The dispersion has a fluorinated surfactantcontent of less than about 300 ppm, preferably less than about 100 ppm,more preferably less than about 50 ppm. In preferred embodiments thenon-melt-processible fluoropolymer particles have an SSG of less than2.30, and more preferably less than about 2.25.

In one preferred embodiment, the five minute foam height of thealiphatic alcohol ethoxylate surfactant is less than about 100 mm, morepreferably less than 50 mm, and most preferably less than about 20 mm.Process advantages for surfactants that have reduced foaming in a fiberspinning composition are discussed below.

In another preferred embodiment, the spinning composition containsfluoropolymer particles comprising a core of high molecular weightpolytetrafluoroethylene and a shell of lower molecular weightpolytetrafluoroethylene or modified polytetrafluoroethylene.

Matrix polymers used in the practice of the present invention may bepolymers containing only hydrogen, carbon, oxygen and nitrogen that aresoluble in aqueous solutions that may be coagulated or precipitated by asalt or a shift of pH. As taught in U.S. Pat. Nos. 3,655,853; 3,114,672;and 2,772,444 cellulose xanthate may be the soluble form of the matrix.However, the use of viscose in fiber forming suffers from seriousdisadvantages related to cost of manufacture, production time andenvironmental hazards. Alternatives to viscose forming have beendeveloped and most recently a process using cellulosic ethers with auniform degree of substitution of the matrix has been fully described inU.S. Pat. Nos. 5,762,846 and 5,820,984.

Cellulosic ether polymers are preferred since these polymers do not meltor soften below the temperature range in which most fluorinated olefinicpolymers melt and the polymer decomposes into carbonaceous material onsintering. For example, such cellulosic polymers are methylcellulose,hydroxyethylcellulose, methylhydroxypropylcellulose,hydroxypropylmethylcellulose, hydroxypropylcellulose, ethylcellulose andcarboxymethylcellulose. The cellulosic ethers preferred for use in thisinvention as a matrix polymer have a uniform degree of substitution, andare soluble in strong aqueous alkali hydroxide, but insoluble in nearneutral pH water. By the term near neutral pH water is meant waterhaving a pH from about 6 to 8. In addition, the matrix polymers used inthe practice of the present invention have no softening or meltingpoint. These polymers decompose at temperatures near the sinteringtemperature of the fiber providing requisite tensile strength until thefluoropolymer particles have coalesced such that the resultantfluoropolymer structure provides the necessary tensile strength.

The structural features that are strongly related to solubility of thecellulosic ethers are the functionality of chemical substituents in thecellulose ethers and the degree of substitution. By degree ofsubstitution (DS) is meant the extent to which the hydroxyl groups of acellulose molecule have been replaced with ether functional groups.

In a cellulose molecule, there are three hydroxyl groups peranhydroglucoside ring. If all three of these hydroxyl groups have beenreplaced, the DS is 3, the maximum degree of substitution.

In order to achieve useful coalesced fluoropolymer fibers, it isdesirable to wash the intermediate fiber structure free of ions absorbedfrom the coagulation bath as well as to remove other impurities such asadditives and/or dispersants that are present in the initialfluoropolymer dispersion and to remove materials that are detrimental tofiber sintering and/or the properties of the final, coalescedfluorinated polymer Fiber.

As used herein, intermediate fiber structure means the extruded andcoagulated mixture of the matrix polymer solution and the polymerparticle dispersion. The intermediate fiber structure of the presentinvention has a self supporting length of at least 30 cm after beingwashed substantially free of ions and impurities. The intermediate fiberstructure produced in the practice of the present invention, afterwashing in near neutral pH water to substantially remove ions andimpurities, shows no substantial loss of strength or integrity, and maybe worked, for example drawn at a modest draw ratio, and sintered toform a final, coalesced fluorinated polymer fiber or shaped article. Theintermediate fiber structure produced by the present invention may beisolated, worked in subsequent processing or used for producing fabricsor batts as is known in this art. The strength of the intermediate fiberstructure an be determined by the procedure described in U.S. Pat. Nos.5,762,846 and 5,820,984.

As will be understood by one skilled in this art, fiber structureincludes, as well as typical fiber monofilament and fiber bundlestructures, elongated tapes, ribbons, Films and the like.

In the practice of the present invention, the composition of theintermediate fiber structure has a cellulosic ether present as a minorconstituent of the fiber solids, while the major constituent is non-meltprocessible fluoropolymer particles having a weight in the intermediatefiber structure that may be from 3 to 20 times that of the matrixpolymer.

In order for the intermediate fiber to be water washable, the matrixpolymer must have precisely defined properties of insolubility in waterwhich is near neutral in pH and at process temperatures. In addition, itis preferred that the matrix polymer neither soften or melt at atemperature substantially below that of sintering, otherwise theintermediate fiber structure may stretch, weaken or break under its ownweight as it is heated to sintering temperatures.

Nonionic cellulosic ethers, such as hydroxypropylcellulose andhydroxyethylcellulose, provide particularly good spinning compositionsfor dispersion spinning of fluorinated polymers. DS values that arerepresentative of the matrix polymers employed in the present inventionare values that range from about 0.02 to 0.5. Uniformity of substitutionfor the matrix polymers employed in the present invention is preferable,and is indicated by transparency of the solution formed in about 10% byweight aqueous sodium hydroxide or the substantial absence of insolublematrix polymer which would collect on a 1 micron pore size filtersuitable for the filtration of diluted matrix polymer solutions ofaqueous sodium hydroxide.

The matrix solution of any of the matrix polymers employed in thepresent invention or mixtures thereof, may be prepared by dissolving theparticular cellulosic ether in a solution of about 5 to 10% by weightsodium hydroxide.

For hydroxypropylcellulose matrix polymer, a material characterized byhaving a viscosity of at least 90 mPa·sec when dissolved at 2% by weightin 10% sodium hydroxide solution and measured at 25° C. is preferred,although solutions of lower viscosity material may be successfully spun.

The spinning compositions used in the process of the present inventionare made by mixing an aqueous dispersion of fluorinated polymerparticles with a solution of the matrix polymer. Aqueous dispersions ofnon-melt processible fluoropolymer particles, such as those describedabove are used in the present process. The solutions of matrix polymershould be clear and of a viscosity that assures good mixing with thedispersion. Preferably the concentration of matrix polymer in thesolution is from 3 to 10% by weight. These components are then mixedsuch that the ratio of the weight of the polymer particles to that ofthe matrix polymer in the intermediate fiber structure is from about 3to 1 to about 20 to 1, and preferably about 9 to 1.

The spinning composition provided by the invention results in manyprocess advantages as described below.

Process

The present invention provides a process for the dispersion spinning ofnon-melt-processible fluoropolymer fiber comprising:

(a) forming a mixture of an aqueous dispersion of non-melt-processiblepolytetrafluoroethylene or modified polytetrafluoroethylene particleshaving an SSG of less than about 2.40, with an aqueous solution of amatrix polymer, the aqueous dispersion containing an aliphatic alcoholethoxylate nonionic surfactant and being essentially free of surfactantscontaining aromatic groups, the aliphatic alcohol ethoxylate having a20% residuals temperature determined by thermogravimetric analysis (TGA)of less than about 300° C., and the dispersion having an fluorinatedsurfactant content of less than about 300 ppm;

(b) extruding the mixture into a coagulation bath containing aconcentration of ions which coagulate the matrix polymer to form anintermediate fiber structure; and

(c) sintering the intermediate fiber structure to decompose the matrixpolymer and coalesce the non-melt-processible fluoropolymer particles toform the fiber.

By the term dispersion spinning is meant the process by which adispersion of insoluble polymer particles is mixed with a solution of asoluble matrix polymer, and this mixture is coagulated by contacting themixture with a coagulation solution in which the matrix polymer becomesinsoluble.

Dispersion spinning is useful for producing fibers fromnon-melt-processible fluoropolymers. These polymers, which are difficultto form by melt extrusion or solution spinning, may be successfully spunfrom a mixture of an aqueous dispersion of fluorinated polymer particlesmixed with a solution of a suitable matrix polymer. An intermediatestructure is formed when this mixture is contacted with a suitablecoagulation bath. Although the intermediate structure is mechanicallysound, a final, sintered structure is generally formed by heating theintermediate structure to a temperature sufficient to coalesce thefluorinated polymer particles. On sintering the matrix polymerdecomposes to form volatile gases and a carbonaceous residue. Oneadvantage of the present invention is that the sintering rolltemperature can be reduced because of the lower 20% residualstemperature of the nonionic surfactant present in the aqueous dispersionof fluoropolymer particles that form part of the spinning composition.Lower sintering roll temperatures result in reduced energy costs.Further, the nonionic surfactant is essentially free of aromatic groupsthat can thermally decompose and be converted to harmful organicaromatic compounds. This results in cleaner air and water emissions andprevents the buildup of tar on sintering rolls. Generally the presentinvention provides a more environmentally clean manufacturing processfor fluoropolymer fiber.

It is preferred to form the fibers in accordance with present inventionby extruding the mixture of the matrix polymer solution and thefluorinated particle dispersion into a coagulation liquid which rapidlygels the article. The formed article may then be washed and furtherprocessed. The composition of coagulation liquids depends, to someextent, on the particular matrix polymer being used. Useful coagulationliquids include a large variety of aqueous solutions typified but notlimited to 40% ammonium acetate-5% acetic acid, 30% acetic acid, or 30%calcium chloride. Of particular value for the cellulose ethers used inthis invention is a 5% sulfuric acid-8% sodium sulfate solution. Thetemperature of the coagulation bath can be adjusted to that whichprovides the best properties for the intermediate fiber structure, andis typically in the range of 20° C. to 90° C.

The intermediate fiber produced in the present invention is preferablywashed substantially free of ions and impurities with no substantialloss of strength. By the term substantially free of ions and impuritiesis meant that the pH and conductivity of deionized wash water isunchanged after dipping the intermediate fiber into the water. The selfsupporting length of the washed intermediate fiber is at least 30 cm.

Another advantage of the spinning composition of the present inventioncontaining an aliphatic alcohol ethoxylate nonionic surfactant having alower 20% residuals temperature and being essentially free of aromaticgroups is that less foam is produced in the wash water. Reduced foamgeneration can be quantified by the five minute foam height test,results of which are shown in Table 2. Five minute foam heights for thealiphatic alcohol ethoxylate surfactants used in this invention aresubstantially lower than alkyl phenol ethoxiate (Triton® X-100).Aliphatic alcohol ethoxylates used in this invention have five minutefoam heights that are preferably less than 100 mm, more preferably lessthan 50 mm and most preferably less than about 20 mm. Less foam resultsin less time-intensive work needed by the fiber processor to control andreduce foam generation. Less foam avoids Fiber spinning breaks and leadsto increased productivity gains.

From the wash step, the fiber is passed over initial tensioning rolls,passed by stationary bars or combs to partially de-water the fiber andthen passed through hot drawing rolls to decompose the matrix polymerand sinter the fluoropolymer. Further drawing, and bleaching steps maybe employed for the manufacture of continuous fiber or a step ofchopping the fiber into short lengths may be employed to produce a PTFEfloc useful as a polymer additive. Fibers produced with the spinningcomposition of the present invention have been found to have betterelongation reducing the frequency of fiber breaks during hot drawing andthereby resulting in better productivity.

Fluoropolymer fibers produced by this invention have any number of usesbut are especially desirable in water repellant garments, filtrationfabrics and when compounded with engineering plastics, high temperatureseals and gaskets. Because of the use of aliphatic alcohol ethoxylatenonionic surfactant having a lower residuals temperature and beingessentially free of surfactants containing aromatic groups, thefluoropolymer fiber is well suited to for compounding with polyacetals.The cleaner burnoff of the surfactants used in the present inventionresults in less residuals that could attack the formaldehyde groups ofthe polyacetal resin. It has been common in past production of fiber tosubject sintered fiber to further oven baking to remove residuals whichare attributable to the phenol groups of alkyl phenol ethoxylates. Thefiber produced from the spinning composition of the present inventionhas no residual phenol groups and an acceptable pH after sintering,thereby providing the ability to eliminate the time-consuming andexpensive oven baking step. The improved fluoropolymer fiber-reinforcedacetal products are especially desired for high temperature seals andgaskets. Polyacetals (sometime referred to as acetal resins) are a classof polyoxymethylene compositions described for example in U.S. Pat. Nos.5,318,813; 5,344,882; and 5,286,807 and commercialized available fromE.I. du Pont de Nemours and Company, Wilmington, Del. under thetrademark DELRIN®.

Methods for Property Determination

Raw Dispersion Properties:

Solids content of PTFE raw (as polymerized) dispersion are determinedgravimetrically by evaporating a weighed aliquot of dispersion todryness, and weighing the dried solids. Solids content is stated inweight % based on combined weights of PTFE and water. Alternately solidscontent can be determined by using a hydrometer to determine thespecific gravity of the dispersion and then by reference to a tablerelating specific gravity to solids content. (The table is constructedfrom an algebraic expression derived from the density of water anddensity of as polymerized PTFE.)

Raw dispersion particle size (RDPS) is measured by photon correlationspectroscopy.

Surfactant Content:

The surfactant and solids content of stabilized dispersion aredetermined gravimetrically by evaporating a small weighed aliquot ofdispersion to dryness following in general ASTM D-4441 but using a timeand temperature such that water but not the surfactant is evaporated.This sample is then heated at 380° C. to remove the surfactant andreweighed. Surfactant content is usually stated in weight % based onPTFE solids.

Resin Properties:

Standard specific gravity (SSG) of PTFE fine powder resin is measured bythe method of ASTM D-4895. If a surfactant is present, it can be removedby the extraction procedure in ASTM-D-4441 prior to determining SSG byASTM D4895.

Melt creep viscosity (MCV) is measured at 380° C. by a modification ofthe tensile creep method disclosed in U.S. Pat. No. 3,819,594, with themold at room temperature, using a molding pressure of 200 kg/cm² (19.6MPa), with the molding pressure held for 2 min, using a load (totalweight suspended from the sample sliver) that varies with the MV toobtain a creep rate suitable for measurement, and waiting at least 30min after application of the load for elastic response to be completebefore selecting viscous response (creep) data for use in thecalculation.

Copolymer Composition:

Comonomer content of the modified PTFE resins is determined by Fouriertransform infrared spectroscopy using the method disclosed in U.S. Pat.No. 4,837,267. For PPVE-modified PTFE, a multiplicative factor of 0.97derived from the calibration curve is used to convert the ratio of theabsorbance at 995 cm⁻¹ to that at 2365 cm⁻¹ to PPVE content in weight %.

20% Residuals Temperature and Thermal Decomposition Temperature:

The 20% residuals temperatures and thermal decomposition temperatures ofsurfactants are determined by thermogravimetric analysis (TGA) using amodified version of ASTM method E-1131 in air. For 20% residualstemperatures, samples to be tested have at least 90% by weightsurfactant content. If the surfactant to be tested contains more than10% by weight water or other volatile solvents, such solvents should beremoved to no more than 10%. Alternatively, to adjust for greater than10% solvent, the residuals weight is recalculated based on the weightfraction of surfactant content in the sample. The samples are heated at10° C./min from room temperature to 204° C. Upon reaching 204° C., theheating rate is reduced to 2° C./min until the samples reach 482° C. At482° C., the sample returns to being heated at 10° C./min until itreaches 600° C. The temperature at which weight loss to a 20% residualsof the original sample is reached is the 20% residuals temperature.

20% residuals temperature and thermal decomposition temperature resultsfor selected nonionic surfactants are summarized in Table 1.

TABLE 1 20% Thermal Sur- Residuals Decom- factant Cloud Tem- positionAlcohol State @ Point perature, Temperature, Surfactant Structure RT °C. ° C. ° C. Genapol ® Branched Liquid 75 274 228 X 080 Serdox ®Primary, Liquid 64 242 220 NBS 6,6 linear Tergitol ® Secondary, Liquid60 — 225 15-S-9 linear Triton ® Alkyl Liquid 65 295 266 X-100 PhenolTergitol ® Secondary, Liquid 65 281 226 TMN-100X branched Tergitol ®Secondary, Liquid 76 260 225 TMN-10 branched Tergitol ® Secondary,Liquid 36 244 223 TMN-6 branched Neodol 1-7 Primary, Liquid 58 250 218linear Tergitol ® is manufactured by Dow Chemical Corporation. Serdox ®is manufactured by by Servo Huls, Delden, the Netherlands. Neodol ® ismanufactured by Shell Chemical. Genapol ® is available from ClariantGmbH. Triton ® is manufactured by Dow Chemical Corporation.Foam Data

The degree of foaming caused by a nonionic surfactant can be quantifiedby the Ross-Miles Foam Test (ASTM D 1173-53, reapproved 2001) in whichdroplets of surfactant are dropped into a column positioned over asurfactant solution of the same surfactant being dropped. Surfactantconcentration is 0.1 wt % surfactant in aqueous solution at 25° C. (77°F.). The bubble height is measured initially after the drop and thenagain in 5 minutes. The measurement after five minutes is referred to asfive minute foam height.

TABLE 2 Initial height Height after 5 min Surfactant Alcohol Structure(mm) (mm) Triton ® X- Alkyl Phenol 160 145 100 Tergitol ® Secondary, 15024 100X branched Tergitol ® Secondary, 113 10 TMN-10 branched Tergitol ®Secondary, 118 5 TMN-6 branched Tergitol ® Secondary, linear 95 2015-S-9

1. A spinning composition useful for the dispersion spinning ofnon-melt-processible fluoropolymer fiber comprising: a mixture of anaqueous solution of a matrix polymer and an aqueous dispersion ofnon-melt-processible polytetrafluoroethylene or modifiedpolytetrafluoroethylene particles having an SSG of less than about 2.40,said aqueous dispersion containing an aliphatic alcohol ethoxylatenonionic surfactant and containing less than about 0.5 weight %surfactants containing aromatic groups, said aliphatic alcoholethoxylate having a 20% residuals temperature determined bythermogravimetric analysis (TGA) of less than about 290° C., and saiddispersion having a fluorinated surfactant content of less than about300 ppm.
 2. The spinning composition of claim 1 wherein the SSG of saidnon-melt-processible fluoropolymer particles is from about 2.14 to about2.40.
 3. The spinning composition of claim 1 wherein the matrix polymeris a cellulosic ether polymer.
 4. The spinning composition of claim 1wherein the matrix polymer is selected from the group consisting ofmethylcellulose, hydroxyethylcellulose, methylhydroxypropylcellulose,hydroxypropylmethylcellulose, hydroxypropylcellulose, ethylcellulose andcarboxymethylcellulose.
 5. The spinning composition of claim 1 whereinthe five minute foam height determined by ASTM D 1173-53 of saidaliphatic alcohol ethoxylate surfactant is less than about 100 mm. 6.The spinning composition of claim 1 wherein the five minute foam heightdetermined by ASTM D 1173-53 of said aliphatic alcohol ethoxylatesurfactant is less than about 50 mm.
 7. The spinning composition ofclaim 1 wherein the five minute foam height determined by ASTM D 1173-53of said aliphatic alcohol ethoxylate surfactant is less than about 20mm.